1. Field of the Invention
This invention relates to the treatment of gypsum plaster, more especially for the purposes of utilization in the production of set gypsum products, more particularly plaster or gypsum board.
2. Description of the Prior Art
Gypsum rock, as it occurs naturally, has a variable content of soluble salts, notably including chloride, sulphate, magnesium and sodium ions. This salt content is carried in over the plaster produced by calcination of such rock and can give rise to problems in the manufacture of gypsum board and like products.
For this reason many deposits of gypsum cannot be used to provide the plaster for gypsum board, although attempts have been made to leach the salt out of the mined gypsum, for example by storing the gypsum in lagoons prior to calcination. This, however, incurs the cost of drying prior to calcination.
While gypsum, calcium sulphate dihydrate, can be immersed in water for prolonged periods without significant chemical change, the cementitious, dehydrated forms, herein referred to as "plaster," and more especially the hemihydrate, react with water below their respective transition temperatures to re-form the hydrated compound. Since a substantially rehydrated material will not set to form a cast product, or will yield only a set product of very low strength, it is clear that the washing of gypsum plaster in its dehydrated forms with water faces a fundamental problem of avoiding unacceptable hydration during the washing process.
One way of avoiding hydration is to conduct the washing operation above the transition point, as in the production of so-called autoclave plaster or in the formation of by-product gypsum in phosphoric acid production, where the calcium sulphate is precipitated in hemihydrate form. However, the cost of heating the large volumes of water necessary for washing the plaster employed in large-scale manufacture, for example of plasterboard, makes this solution of the problem uneconomic.
British Pat. Nos. 1 127 397 (Bird Machine Company) and 1 190 014 (Imperial Chemical Industries) are examples of the use of continuous hot centrifigation to separate liquid from suspensions of calcium sulphate hemihydrate without setting or hydration, although in either case in this done subsequent to a washing or leaching operation. In the first patent a slurry of hemihydrate is discharged from an autoclave into the centrifuge at about the atmospheric boiling point of water and the solids discharge compartment of the centrifuge casing is also maintained above the inversion temperature. In the latter patent a similar slurry is dewatered in a centrifuge in which all the surfaces on which hemihydrate is liable to be deposited are maintained at a temperature of at least 100.degree. C.
Schoch et al in U.S. Pat. No. 2 285 098 discloses that although below the invention temperature, hydration will occur, it may not be sufficiently rapid to prevent successful filtration and washing of the hemihydrate. He teaches, however, that the temperature should not fall to any extent below 80.degree. C. in order to give ample time for these operations.
Hydration can also be delayed by the addition of a setting retarder, so allowing more time for the washing process without an undesirable degree of hydration, but not only is the use of a retarder more costly in itself, but also has the disadvantage that the setting of the final cast product is correspondingly affected. This is particularly undesirable in continuous production processes, such as plasterboard manufacture, where rapid setting is desirable in order to shorten the length of the production lines. Accelerators can be used to increase the setting rate, but their use to offset a decrease in setting rate brought about by a setting retarder adds yet another undesirable element of cost.